1. Field of the Invention
The present invention relates to a photoacid generator added to a positive photosensitive composition for use in a process for producing a semiconductor such as an IC, a process for producing a circuit board for e.g., a thermal head or a liquid crystal, and other photofabrication processes, and relates to a positive photosensitive composition containing the photoacid generator and a method of forming a pattern by use thereof. More particularly, the present invention relates to a photoacid generator that finds appropriate application when an exposure light source emits far ultraviolet, electron beams or the like of wavelength 250 nm or shorter, preferably 220 nm or shorter, and also relates to a positive photosensitive composition containing the photoacid generator and a method of forming a pattern by use thereof.
2. Description of the Related Art
When the conventional resist comprised of a novolak and a naphthoquinone diazide compound is used in the lithographic patternization employing far ultraviolet or excimer laser beams, the light transmission to the bottom portion of the resist is hampered by the high absorption in the far-ultraviolet region of the novolak and naphthoquinone diazide with the result that only a tapered pattern can be obtained with low sensitivity.
One of the means for solving this problem can be provided by a chemical-amplification resist composition. A chemical-amplification positive resist composition is a pattern forming material that is capable of, upon exposure to far ultraviolet or other radiation, generating an acid at the exposed area and, by a reaction catalyzed by the acid, changing the solubility in a developer between the area having been exposed to actinic radiation and the nonexposed area to thereby attain pattern formation on a substrate.
As examples of such resist compositions, there can be mentioned combinations of a compound capable of generating an acid by photodecomposition with any of an acetal or O,N-acetal compound, an orthoester or amide acetal compound, a polymer having an acetal or ketal group in its principal chain, an enol ether compound, an N-acyliminocarbonic acid compound, a polymer having an orthoester group in its principal chain, a tertiary alkyl ester compound, a silyl ester compound and a silyl ether compound. These exhibit a quantum yield of, in principle, over 1, thereby ensuring a high photosensitivity.
Further, as a system that is decomposed by heating in the presence of an acid to thereby become soluble in an alkali, there can be mentioned, for example, systems each consisting of a combination of a compound capable of generating an acid upon exposure to radiation with any of an ester of tertiary or secondary carbon (for example, t-butyl or 2-cyclohexenyl) or carbonic acid ester compound, an acetal compound and a t-butyl ether compound, as described in non-patent references 1 to 5.
In these systems, a resin whose fundamental skeleton consists of a poly(hydroxystyrene) exhibiting a low absorption mainly in the region of 248 nm is employed as a major component thereof. Accordingly, when a KrF excimer laser is used as a exposure light source, there can be attained a high sensitivity, high resolving power and favorable pattern formation. Thus, a system superior to the conventional naphthoquinone diazide/novolak resin system can be realized.
However, in using a light source of a further shorter wavelength, for example, an exposure light source of an ArF excimer laser (193 nm), as the compounds having aromatic groups inherently exhibit a sharp absorption in the region of 193 nm, the above-mentioned chemical amplification system has not been satisfactory. Moreover, the use of a poly(meth)acrylate as a polymer exhibiting a low absorption in the wavelength region of 193 nm is described in non-patent reference 6. However, there is the problem that this polymer is poor in the resistance to dry etching as generally carried out in semiconductor producing processes as compared with that of the conventional phenolic resin having an aromatic group.
In this connection, non-patent reference 7 reports that polymers having an alicyclic hydrocarbon group not only exhibit a resistance to dry etching comparable to that realized by an aromatic group but also have a low absorption in the region of 193 nm. Hence, in recent years, intensive studies have been conducted on the use of such polymers.
The photoacid generators that can be used along with the above-mentioned polymers include a compound capable of generating a trifluoromethanesulfonic acid, such as triphenylsulfonium trifluoromethanesulfonate, and a compound capable of generating a fluoroalkylsulfonic acid with a prolonged chain.
Further, the well-known acid generators include those capable of generating a perfluoroalkanesulfonic acid, such as triphenylsulfonium triflate or bis(t-butylphenyl)iodonium perfluorobutanesulfonate. Generally, perfluoroalkyl compounds have such a high hydrophobicity that appropriate use can be made in, for example, the water repellent finishing of clothes. Accordingly, the resists containing an acid generator that when exposed to actinic rays, generates a perfluoroalkylsulfonic acid exhibit poor affinity to aqueous developers, thereby causing the problem that a sensitivity decrease or occurrence of development residue (scum) would be invited by a deterioration of developability. As measures for coping with this problem, it has been reported to use a fluoroalkanesulfonic acid having an amido structure in its molecules (see, for example, patent reference 1) or a fluoroalkanesulfonic acid having an ester structure in its molecules (see, for example, patent reference 2).
Patent reference 1: Jpn. Pat. Appln. KOKAI Publication No. (hereinafter referred to as JP-A-) 2002-214774,
Patent reference 2: JP-A-2008-170983,
Non-patent reference 1: Polym. Eng. Sce., Vol. 23, page 1012 (1983),
Non-patent reference 2: ACS. Sym. Vol. 242, page 11 (1984),
Non-patent reference 3: Semiconductor World, 1987, November Issue, page 91,
Non-patent reference 4: Macromolecules, Vol. 21, page 1475 (1988),
Non-patent reference 5: SPIE, Vol. 920, page 42 (1988),
Non-patent reference 6: J. Vac. Sci. Technol., B9, 3357 (1991), and
Non-patent reference 7: Proc. of SPIE, 1672, 66 (1992).